A liquid crystal display device has two substrates. One is a TFT substrate in which pixel electrodes, thin film transistors (TFTs), and the like are formed in a matrix form. The other is a counter substrate disposed opposite to the TFT substrate, in which color filters and the like are formed at positions corresponding to the pixel electrodes of the TFT substrate. A liquid crystal is interposed between the TFT substrate and the counter substrate. Thus, the liquid crystal display device forms an image by controlling the transmittance of light of the liquid crystal molecules for each pixel.
Along with the demand for reducing the overall size of the liquid crystal display device as a set, there is also a strong demand for reducing the thickness of a liquid crystal display panel of the liquid crystal display device, while the size of the screen remains unchanged. A thin liquid crystal display panel is obtained in such a way that a liquid crystal display panel is produced, and then the outside of the liquid crystal display device is polished to a desired thickness.
The TFT substrate including pixel electrodes, TFTs, and the like, and the counter substrate including color filters, are glass substrates constituting the liquid crystal display device. The two glass substrates are standardized, for example, at a thickness of 0.5 mm or 0.7 mm. It is difficult to obtain such standardized glass substrates from the market. Further, very thin glass substrates have a problem in the production process in terms of mechanical strength and bending or deformation, which reduces the production yield. As a result, a liquid crystal display panel is formed from the standardized glass substrates, and then the outside of the formed liquid crystal display panel is polished to a desired thickness.
When the thickness of the liquid crystal display panel is reduced, the problem of the mechanical strength arises. There is a risk that when a mechanical pressure is applied to a surface of the liquid crystal display panel, the liquid crystal display panel will be destroyed. In order to prevent this, a front window is attached to the screen side of the liquid crystal display panel when the liquid crystal display panel is set into a mobile phone and the like.
The front window may be provided at a distance from the liquid crystal display panel so that the external force applied to the front window has no influence on the liquid crystal display panel. However, such a configuration has a problem that reflection occurs in the interface of the front window, reducing the quality of the image.
In order to solve this problem, for example, JP-A No. 174417/1999 describes a technology in which an adhesive elastomer is interposed between the front window and the liquid crystal display panel. The adhesive elastomer helps not only to protect the liquid crystal display panel from the external force but also to prevent the reflection in the interface of the front window by setting the refractive index of the adhesive elastomer to a value close to the refractive index of the front window.
The liquid crystal display device can have a small size and a reduced thickness, allowing it to be used in various types of applications such as mobile phones. Recently, mobile phones are provided with various applications. Further, input devices are expected to have a function that allows finger input through a touch panel, in addition to the conventional key button operation. In this case, the touch panel is attached to the side of the counter substrate of the liquid crystal display panel.
Even in the liquid crystal display device of such a configuration, the demand for a reduction in the thickness of the liquid crystal display panel still exists. Also the thickness of the touch panel provided on the liquid crystal display panel is expected to be reduced. Thus, even if the touch panel is present, the problem of the mechanical strength of the liquid crystal display panel still remains.
In order to solve such a problem, a window formed from glass or resin is mounted on the upper side of the touch panel. In this case also, when an air layer is present between the liquid crystal display panel and the touch panel, or between the touch panel and the front window, the quality of the image is reduced by the interface reflection. In order to prevent this, JP-A No. 83491/2008 describes a configuration in which an adhesive layer or anti-reflection coating is formed between the liquid crystal display panel and the touch panel, or between the touch panel and the front window.
A liquid crystal display panel can be protected from an external force by providing a front window. However, the strength of the liquid crystal display panel is very different depending on the position of the liquid crystal display panel. In other words, the liquid crystal display panel is formed in such a way that a TFT substrate and a counter substrate are bonded at a peripheral portion thereof by a seal material, with a liquid crystal interposed between the TFT substrate and the counter substrate. The TFT substrate is formed larger than the counter substrate. A terminal portion is formed in the portion of the TFT substrate extending further than the counter substrate, to which an IC driver is mounted and a flexible wiring substrate is attached. Further, an upper polarization plate is bonded to the upper side of the counter substrate while a lower polarization plate is bonded to the lower side of the TFT substrate, which is provided corresponding to the portion to which the upper polarization plate is bonded.
In order to meet the demand for reduction in the thickness of the liquid crystal display panel, the TFT substrate and the counter substrate are polished to a thickness of about 0.15 mm. Each of the upper and lower polarization plates has a thickness of about 0.13 mm. The polarization plate is formed from a resin which is stronger than the glass. Thus, the strength of the portion in which the counter substrate and the TFT substrate are bonded to each other is equivalent to the total thickness of the two glasses and the two polarizations. In contrast, the thickness of the portion of only the TFT substrate is 0.15 mm. Thus, the strength of the portion of only the TFT substrate is one fourth or less of the strength of the portion in which the TFT substrate and the counter substrate are bonded to each other.
Because the liquid crystal display panel has such a configuration, when an external force is applied to the liquid crystal display panel, the stress is concentrated in the portion of only the TFT substrate, in particular at the boundary between the area in which the TFT substrate and the counter substrate overlap with each other and the area of only the TFT substrate. As a result, a crack occurs in the TFT substrate at the boundary region. This problem is encountered both during and after production process of the liquid crystal display panel.